This invention relates to atomic frequency oscillators, and more particularly to an atomic frequency oscillator of the type having a resonance cell placed in a microwave resonator surrounded by an electromagnetic shield, a laser module situated outside the electromagnetic shield and emitting a light beam which reaches the resonance cell, the light beam being used for optically pumping a gas into the resonance cell.
U.S. Pat. No. 5,387,881 to the present assignee describes an atomic frequency oscillator. This patent may be referred to for a detailed study of the operating principle of such oscillators, only those features necessary for understanding the present invention being mentioned here. FIG. 1 of the cited reference illustrates the design of such a device. They generally include a light source 13, a microwave resonator 15, and optical sensor means 17. An electromagnetic shield 20b surrounds the cavity of the microwave resonator; on the one hand, it keeps outside magnetic fields from interfering with the resonator, and on the other hand, it keeps the resonator itself from interfering with other components, such as the light source or the optical sensor means. The cavity of the microwave resonator contains a resonance cell 16 containing a gas, e.g., a rubidium or cesium vapor. A quartz oscillator associated with a frequency multiplier 11 generates the signal for exciting the microwave resonator. The light source generates a light beam which reaches the resonance cell and, by optical pumping, produces a population reversal between the hyperfine levels F=1 and F=2 of the ground state of the rubidium atoms. The resonance cell becomes practically transparent when all the atoms have reached the upper hyperfine level F=2. It is equally possible to carry out the optical pumping in such a way that the population reversal takes place toward the lower hyperfine level F=1. Then, by means of the oscillator, a frequency corresponding to the transition energy between the hyperfine levels is applied to the cavity of the microwave resonator, i.e., a frequency close to 6835 MHz in the case of rubidium. The result is a hyperfine transition, so that the population reversal is destroyed. The cell becomes more opaque again. The quantity of light reaching the optical sensor means is therefore minimal when the frequency of the oscillator, after multiplication, corresponds to the transition frequency. In this way, the oscillator is controlled by the very stable and well-defined frequency corresponding to the separation energy of the hyperfine levels.
In the example of the above-mentioned U.S. patent, the light source used is a lamp, shown in FIG. 2, containing a mixture of rubidium and argon, and placed within an exciting coil. This arrangement is complicated and expensive. Furthermore, it takes up a great deal of space, and the lamp must necessarily be placed near the absorption cell. Thus these two elements interfere with each other, and a grid 51 must be provided to limit such interference. What is more, the energy output of this device is poor. Hence this design is hardly suitable to applications in which cost and size pay an important part, and in which the cell must be insulated as effectively as possible from out side interference. It has therefore been sought in the prior art to replace the light source by a source not having these drawbacks, e.g., by a laser diode.
Japanese Laid-Open Application No. 3,078,319 (Anritsu Corporation) describes another atomic frequency oscillator using a laser diode with stabilized frequency as the light source. In FIG. 1b in particular, in element 24, the beam of the laser diode is seen to be separated by means of a beam-splitter into a measuring beam and a control beam. The control beam reaches an optical resonator 24d, and a photosensor 24e supplies a signal used by control means 24f, 24g, 24h, 24i to control the current injection of the laser diode so that it emits precisely at the resonance frequency of the optical resonator 24e.
There are other systems for stabilization of laser diodes in which the emission frequency is controlled by the absorption frequency of a cell containing a gas, e.g., rubidium vapor, selectively attenuating certain wavelengths. The emission of the laser diode can then be adjusted so that the intensity of the beam passing through the cell is minimal, which amounts to locking up the emission frequency to the peak absorption frequency of the cell.
However, the system disclosed in above-mentioned Japanese Laid-Open Application No. 3,078,319 of Anritsu has a number of drawbacks making its use problematic. Part of the light beam emitted by the laser diode is reflected by the surface of the beam-splitter and returned toward the laser diode. This feedback of light into the laser diode modifies certain emission characteristics, such as the wavelength or the mode of emission, and therefore makes stabilization very difficult. Several solutions have been proposed for reducing, but not eliminating, the undesirable feedback: by treating the faces of the beam-splitter with antireflective coatings, by disaligning the beam-splitter in relation to the optical axis, or by using an optical insulator. Although such operations do improve the device, they entail complications and additional costs. Furthermore, upon temperature fluctuations, the distance between the laser diode and the interface of the beam-splitter changes because of the dilation. The phase and/or amplitude of the light returned to the laser diode therefore depends upon the temperature, making it difficult to predict how the device will perform. In addition, the positioning of the various components is complicated in this design. The absorption cell, which may be quite large in volume according to the chosen gas, must be positioned in the optical path of the light beam, i.e., near the laser diode and the measuring circuit. If the laser diode is placed on a printed circuit, it may be difficult to fix the absorption cell there and to adjust it correctly.
Moreover, in this design the laser diode, the components of the atomic oscillator, and the photosensor 24e are inadequately protected against the electromagnetic interference caused mutually and from the outside. Even putting an electromagnetic shield around the microwave resonator would solve the problem only incompletely since this shield would have to have openings a few millimeters in diameter to allow the laser beam coming from the laser diode to enter, then leave. Thus, the shield could perform its task only very insufficiently.
Devices other than atomic frequency oscillators have naturally been proposed in which the light beam necessary for controlling a laser diode is taken off by means of an optical fiber placed directly in front of the laser diode. European Patent No. 0 479 118 to Dornier describes such a device. However, it uses a single-mode optical fiber 8, difficult to put to work and requiring precautions for the alignment with the laser diode. Moreover, a single-mode fiber is not capable of conveying a light beam with the required conditions of stability. It is, in fact, very difficult to keep coupling losses stable with a single-mode fiber. Hence this device is suitable only for applications in which the stability demanded of the operating light beam is not very critical--in this patent, for fiber optics sensor devices of the type described. Nowhere in this reference is it suggested that the device might also be adapted to an atomic frequency oscillator, nor how it would then be necessary to modify it.
The feedback problem is not at all solved by the foregoing patent to Dornier. This solution therefore can not easily be applied to an atomic frequency oscillator, for in order to effect the separation between the operating light beam and the controlling light beam, it is necessary in this design, too, to provide a beam-splitting element, here in the form of a fiber optics coupler 10. However, this type of coupler is at least as difficult to produce as a beam-splitter. The light entering the prior art couplers passes through an optical interface which returns part of the light toward the laser diode 2 through the optical fiber.
Furthermore, coupling a single-mode fiber to a Fabry-Perot cavity 12 as is done generates a very great feedback. Hence it is not possible to use the teaching of this patent for an atomic frequency oscillator, in which abrupt changes of the mode of emission of the laser diode are absolutely catastrophic. Those skilled in the art, wishing to solve the feedback problems mentioned in connection with the Anritsu reference (Japanese No. 3,078,319), would therefore be rather dissuaded by this patent to Dornier (European Patent No. 0 479 118) from using an optical fiber in the case of an atomic frequency oscillator, aside from the fact that the device described there would then have to be greatly modified.
In the above-mentioned design of Anritsu, the beam-splitter separates the light beam emitted by the laser diode into two beams--a measuring beam and a controlling beam--of substantially equal intensity. Now, the control means can generally do with a small fraction, typically about 2 or 3%, of the light intensity required by the atomic frequency oscillator, whereas in this design they receive the same intensity. The laser diode must therefore operate at needlessly high power, thus reducing its life span, hastening aging, generating unwanted heating, and wasting energy.
It will be noted that the problem of the power to be furnished to the laser diode is not solved by the above-mentioned patent to Dornier any more than by the Anritsu proposal. Conventional fiber optics couplers in fact split the incoming light beam into two beams of equal intensity. Consequently, the control means receive a light intensity equal to the rest of the device, even though they could do with a fraction of that intensity in many applications.
For several years now, owing to the advent of compact disks and laser printers, for example, laser diodes are mass-produced on a large scale and have therefore become very economical. In certain fields of application, they are even increasingly replacing conventional gas lasers owing to their great advantages of miniaturization, long life, efficiency, and ease of execution.
Thus, there are currently a large number of different types of laser diodes, e.g., double-heterojunction diodes, DBR (distributed Bragg reflector) diodes, vertical-cavity diodes, etc., corresponding to a large number of different needs.
Double-heterojunction laser diodes of the AsGaAl type, for instance, find use in a great many different applications. They emit laser light with a wavelength between 750 nm and 880 nm, close to the visible spectrum and adapted to the usual silicon photoelectric receivers. The light frequency emitted by this type of diodes depends upon two parameters:
the injection current causes the frequency to vary by 3 GHz/mA, or 0.006 nm/mA; PA0 the temperature causes the frequency to vary by 30 GHz/.degree. C., or 0.06 nm/.degree. C.
For an application in an atomic frequency oscillator, it is necessary to have a laser source emitting at an absolutely stable frequency. The emission frequency must therefore be stabilized by controlling the injection current and/or the temperature of the laser diode. Steady current sources can be designed producing a current of 150 mA with an accuracy of .+-.5 .mu.A. If this current is applied to a laser diode, the frequency inaccuracy therefore will be about df/f=5.multidot.10.sup.-8. Over a long period of time, it is difficult to guarantee a more accurate temperature of the diode than .+-.1 mK. This corresponds to a df/f error of 8.multidot.10.sup.-8.
For atomic frequency oscillators, the accuracy yielded by the foregoing means proves to be insufficient. Moreover, even if it were possible to maintain a sufficiently constant current and temperature, variations due to aging of the laser diode could not be compensated for with this method. It is therefore necessary to stabilize the laser diode with the aid of an outside reference element.